🐸’s review published on Letterboxd:
"Chapter One : The Tangent Universe.
The Primary Universe is fraught with great peril. War, plague, famine and natural disaster are common.
Death comes to us all. The Fourth Dimension of Time is a stable construct, though it is not impenetrable.
Incidents when the fabric of the fourth dimension becomes corrupted are incredibly rare.
If a Tangent Universe occurs, it will be highly unstable, sustaining itself for no longer than several weeks.
Eventually it will collapse upon itself, forming a black hole within the Primary Universe capable of destroying all existence."
In this case, the Tangent Universe would be the one in which almost the entirety of the movie happens. Frank tells Donnie that the whole will end on october 30th, because this is the date the Tangent Universe (that, as mentioned above, can sustain itself for no longer than several weeks) will collapse. But the collapsing of the Tangent Universe would also destroy the Primary Universe, which is why Donnie is led through this "mission" to save the world.
"Chapter Two: Water and Metal
Water and Metal are the key elements of Time Travel.
Water is the barrier element for the construction of Time Portals used as gateways between Universes as the Tangent Vortex.
Metal is the transitional element for the construction of Artifact Vessels."
This one is very simple. The "artifact" that allows the Time Portal to be created at the begining of the movie is a part of a plane, which happened to get off during a storm. So, water and metal both helped create the portal into the Tangent Universe.
"Chapter Four: The Artifact And The Living
When a Tangent Universe occurs, those living nearest to the Vortex, will find themselves at the epicenter of a dangerous new world.
Artifacts provide the first sign that a Tangent Universe has occurred.
If an Artifact occurs, the Living will retrieve it with great interest and curiosity.
Artifacts are formed from metal, such as an Arrowhead from an ancient Mayan civilization, or a Metal Sword from Medieval Europe.
Artifacts returned to the Primary Universe are often linked to religious Iconography, as their appearance on Earth seems to defy logical explanation.
Divine intervention is deemed the only logical conclusion for the appearance of the Artifact."
As it is said in the movie, nobody can explain how or why this artifact got into Donnie's room and it becomes an object of mystery to pretty much everyone.
"Chapter Six: The Living Receiver
The Living Receiver is chosen to guide the Artifact into position for its journey back to the Primary Universe.
No one knows how or why a Receiver will be chosen.
The Living Receiver is often blessed with a Fourth Dimensional Powers.
These include increased strength, telekinesis, mind control, and the ability to conjure fire and water.
The Living Receiver is often tormented by terrifying dreams, visions and auditory hallucinations during his time within the Tangent Universe.
These surrounding the Living Receiver, known as the Manipulated, will fear him and try to destroy him."
So basically the Living Receiver is Donnie, as simple as this. At the end of the movie, Donnie "sends" the engine of the plane his mother and sister were on (the Artifact) in the Time Portal, allowing him to save both universes.
"Chapter Nine: Ensurance Trap
The Manipulated Dead will set an Ensurance Trap.
The Living Receiver must ensure the fate of all mankind."
I am not certain what the Ensurance Trap would be if not the "mission" Donnie has.
"Chapter Ten: The Manipulated Dead
The Manipulated Dead are more powerful than the Living Receiver. If a person dies within the Tangent Dimension, they are able to contact the Living Receiver through the Fourth Dimensional Construct.
The Fourth Dimensional Construct is made of Water.
The Manipulated Dead will manipulate the Living Receiver using the Fourth Dimensional Construct (see Appendix A and B).
The Manipulated Dead will often set an Ensurance Trap for the Living Receiver to ensure that the Artifact is returned safely to the Primary Universe.
If the Ensurance Trap is successful, the Living Receiver is left with no choice but to use his Fourth Dimensional Power to send the Artifact back in time into the Primary Universe before the Black hole collapses upon itself."
The most important element here is to understand that Frank is the Manipulated Dead. The rest is Donnie's mission, which achieved only with the help of Frank, the boy Donnie kills toward the end of the movie.
"Chapter Twelve: Dreams
When the Manipulated awaken from their Journey into the Tangent Universe, they are often haunted by the experience in their dreams.
Many of them will not remember.
Those who do remember the Journey are often overcome with profound remorse for the regretful actions buried within their Dreams, the only physical evidence buried within the Artifact itself, all that remains from the lost world.
Ancient myth tells us of the Mayan Warrior killed by an Arrowhead that had fallen from a cliff, where there was no Army, no enemy to be found.
We are told of the Medieval Knight mysteriously impaled by sword he had not yet built.
We are told that these things occur for a reason."
This is the reason why, at the end of the movie, we see Frank touching his eye (where he's been shot in the Tangent Universe), Gretchen waving at Donnie's mother, etc. These people have brief memories of the Tangent Universe, has if it was a dream they had.
The movie starts with the creation of the Tangent Universe. Frank, telling Donnie to wake up and leading him out of the house, saves him so Donnie can himself save the universe. What almost kills Donnie is a plane engine, which is made of metal and comes from god knows where, caracteristics that perfectly fit the Philosophy of Time Travel description of an artifact. These are the signs that the Tangent Universe has been created.
Donnie being the Living Receiver, he is given some tasks and powers to save both universes (flooding the school to created a portal with water, being able to sends the plane engine into the time portal, etc.)
Frank, having been killed in the Tangent Universe, is able to become a Manipulated Dead and help Donnie through his quest. He wakes Donnie up so he doesn't get killed, he helps Donnie flood the school so he can meet Gretchen and ask her out, he helps Donnie burn Jim Cunningham's house so he gets arrested and Donnie's mom has to take the plane (which will open the Time Portal) with his sister, etc. By doing so, Frank is also saving himself. By the end of the movie, Donnie's girlfriend is dead, he has killed a man, and has no other choice but to sacrifice himself and go through the Time Portal to save the world. He would have no interest to not do so.